slave: don't fail if user session already active when activating from unlock
    
    After GDM successfully reauthenticates a user it runs
    
    gdm_slave_switch_to_user_session
    
    to jump to the user's VT and unlock it.  Of course, if the user
    is reauthenticating from an unlock screen instead of a user
    switched login screen, then they'll already be on the right VT.
    
    gdm_slave_switch_to_user_session fails if the user is already
    switched to the session specified.  This failing behavior is used
    at login time to decide start a user's session (versus switching
    to a user's already running session).
    
    This commit changes gdm_slave_switch_to_user_session to take
    an additional parameter to decide whether or not switching to
    an already active session should fail, or merely skip to the
    unlocking step.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=685988
    (cherry picked from commit 99c4f960c0689d310a799e87f6361fb25a8862f6)